**Core Concept**
Spleen rupture is a life-threatening condition requiring prompt diagnosis and management. The investigation of choice should be able to quickly and accurately assess splenic injury, detect bleeding, and guide immediate surgical intervention if necessary.

**Why the Correct Answer is Right**
CT scan is the investigation of choice for diagnosing splenic rupture due to its high sensitivity and specificity in detecting splenic injuries, including lacerations, hematomas, and vascular injuries. CT scan can rapidly assess the extent of injury and detect bleeding, which is critical in guiding surgical management. The multi-planar imaging capabilities of CT scan allow for accurate evaluation of the spleen's relationship with surrounding structures, such as the diaphragm and bowel, which is essential for surgical planning.

**Why Each Wrong Option is Incorrect**
**Option A:** Peritoneal lavage is a useful diagnostic tool for detecting intraperitoneal bleeding but is not the investigation of choice for diagnosing splenic rupture. It can be used as a bedside tool in emergency settings but is less accurate than CT scan in detecting splenic injuries.

**Option B:** Ultrasound is a valuable tool for evaluating abdominal trauma, particularly in pediatric patients and in situations where CT scan is not readily available. However, it may not provide a comprehensive evaluation of the spleen's relationship with surrounding structures, limiting its utility in guiding surgical management.

**Option D:** MRI is not typically used as an emergency investigation for splenic rupture due to its longer scanning time, limited availability, and higher cost compared to CT scan. While MRI can provide detailed images of the spleen, it is not the preferred choice for acute trauma evaluation.

**Clinical Pearl / High-Yield Fact**
In cases of suspected splenic rupture, a CT scan with IV contrast should be performed as soon as possible to accurately assess the extent of injury and guide immediate surgical intervention. The "CT scan" should be performed with IV contrast to help identify active bleeding and to evaluate the spleen's relationship with surrounding structures.

**✓ Correct Answer: C. CT scan**